The performance of gas turbine engines, whether measured in terms of efficiency or specific output, is improved by increasing the turbine gas temperature. In modern engines, the high pressure (HP) turbine gas temperatures are hotter than the melting point of the material of the blades and vanes, necessitating internal air cooling of these airfoil components. During its passage through the engine, the mean temperature of the gas stream decreases as power is extracted. Nonetheless, in some engines, the intermediate pressure (IP) and low pressure (LP) turbines are also internally cooled.
FIG. 1 shows an isometric view of a typical single stage cooled turbine. Cooling air flows are indicated by arrows.
Internal convection and external films are the prime methods of cooling the airfoils. HP turbine nozzle guide vanes 1 (NGVs) consume the greatest amount of cooling air on high temperature engines. HP blades 2 typically use about half of the NGV flow. The IP and LP stages downstream of the HP turbine use progressively less cooling air.
The HP turbine airfoils are cooled by using high pressure air from the compressor that has by-passed the combustor and is therefore relatively cool compared to the gas temperature. Typical cooling air temperatures are between 800 and 1000 K, while gas temperatures can be in excess of 2100 K.
The cooling air from the compressor that is used to cool the hot turbine components is not used fully to extract work from the turbine. Therefore, as extracting coolant flow has an adverse effect on the engine operating efficiency, it is important to use the cooling air effectively.
In order to obtain high turbine stage efficiency it is also important to control the clearance between the rotor blade tip and the facing stationary shroud segment or seal liner 3, particularly at cruise condition. An effective means of controlling this gap is to employ a shroud 4 on the tip of the blade.
However, as turbine gas temperatures rise, it becomes difficult to maintain the structural integrity of the shroud. One approach is to use relatively large quantities of cooling air to both cool the shroud and to locally dilute the gas temperature coming from the combustor. However, in the case of the coolant used to actively cool the shroud, there is a limit to the amount of coolant that can be physically passed through this relatively slender structure.